PRRSV belongs to the family Ateriviridae, one of animal RNA virus families. Antigenic properties of PRRS viruses, like other RNA viruses, continually change, which results in a most problematic issue in developing an effective vaccine against this disease causing agent. However, there are fundamentals that are not changed in the PRRSV biological system. Importantly, the virus infects a host cell of a multicellular organism for its replication or growth. To infect, the virus must attach to a host cell as part of its life cycle. For attachment, the virus must have a viral receptor recognition protein (RRP) that recognizes one or more specific receptors on the host cell. Last, the host cell's receptor generally does not change because it is usually required for a particular function and so not intended for virus recognition.
But a virus utilizes the cell's receptor to attach or recognize the host cell. Rather than modifying the receptor structure, an organism containing the host cell may produce antibodies that recognize the RRP of the virus to block attachment of virus to the host cell. The antibodies are commonly referred to as neutralizing antibodies (NA). In response, a population of virus often contains or produces modifications to its RRP that allow escape from NA recognition. However, the modifications to the RRP are limited by the fact that the modified RRP must still recognize the cellular receptor for virus attachment. If a modification results in a non-functional RRP, the virus cannot attach, and so cannot replicate or survive.